The invention relates to a zero axis crossing detector for bipolar circuits made up of two current paths provided with transistors and connected in parallel so that a d.c. current flows in one half-wave of the alternating current signal via one current path and a direct current flows in the other half-wave via the other current path, and both current paths are blocked only during the zero axis crossing phases of the alternating current signal.
Zero axis crossing detectors are used for example for controlling full-wave thyristors (Triac). The triac should be controlled during the zero axis crossing of an alternating current signal in order to reduce to a minimum the disturbances caused by switching which occur in the alternating current network. Moreover, zero axis crossing detectors are also required for phase gating control circuits for synchronizing saw-tooth generators.
Until now a zero axis crossing detector was used in which a first transistor is connected by its base electrode to the signal input for the alternating current signal. The emitter-collector path of the first transistor is connected between the poles of a direct voltage source together with a collector-resistor.
Connected in parallel to the collector-emitter path of the first transistor is the emitter-collector path of a second transistor, in which the collector resistor of the first transistor forms the emitter resistor of the second transistor. The base of this second transistor is connected to the collector of a third transistor, the emitter of which is connected to the signal input. The base of the third transistor is connected to earth. The first and third transistors are pnp transistors while the second transistor is an npn transistor.
The described prior art circuit has the substantial disadvantage that the output pulses are non-symmetrical in their parts allotted to the positive and negative half-wave of the alternating current signal. This rests on the fact that the current which is required for connecting one current path is substantially smaller than the current required for connecting the other current path. Thus the first transistor is driven in emitter connection in the negative half-wave so that the total current amplification of the first transistor goes into the drive process. On the other hand, the third transistor is driven in base connection in the positive half-wave, this base connection not providing any current amplification. As a result it is compulsory that the pulse width is distributed non-symmetrically over the two half-waves of the alternating current signal.
The lack of symmetry is undesirable in many cases